The present invention relates to a novel heat-resistant inorganic composition useful as a coating material which is capable of being firmly bonded to a metal surface, such as iron, copper, silver and the like, or to an inorganic material surface, such as inorganic fibers, refractory ceramics, glasses and the like. The novel heat-resistant inorganic composition is also useful as an adhesive agent for bonding metal articles or for bonding articles of a metal and an inorganic material, such as the metals and inorganic materials described above. More particularly, the invention relates to a heat-resistant inorganic composition which is practically useful and is capable of exhibiting excellent adhesion without peeling at elevated temperatures over a wide range of from 100.degree. to 800.degree. C. Further, the heat-resistant inorganic composition of the present invention has a high workability and can easily be used for coating or bonding.
In the prior art, various types of heat-resistant coating compositions have been used for providing high temperature protection for the surface of metallic materials which contact corrosive liquids and gases or molten metals. Heat-resistant coating compositions have also been used as adhesives for providing heat insulating layers of ceramic fiber blocks on the walls of furnaces; and as anti-corrosion coating materials coated on steel-made ducts for exhaust gases at high temperatures and drain pipes; as well as anti-oxidation coating materials coated on the steel-made heat reflectors and heat shields in refuse incinerators. However, only a few known compositions are satisfactory in heat resistance. Among these, those formulated using silicone resins as the binder are relatively superior, although they have the defect that their usability is limited. These silicone-based coating compositions are limited to uses at temperatures of 100.degree. to 200.degree. C. at the highest, because the durability thereof is greatly decreased when used at a temperature higher than 200.degree. C. by rapid degradation of the composition.
There are also known methods for improving the heat resistance of metal articles by coating them with a glaze, and then firing to form an enamel lining as in an enameled ironware. Alternatively, metal articles can be coated with a layer of an inorganic oxide by spray fusion. These methods are, however, disadvantageous due to the difficulties in their coating procedures, and usually the adhesive bonding of a metal, such as iron, copper, silver and the like, and an inorganic material, such as ceramics, ceramic fibers, glasses and the like, is performed by use of a solder glass.
Soldering with a solder glass, however, requires a heat treating at a temperature of 400.degree. to 500.degree. C. or even higher and, therefore, from a practical point of view, such a method is hardly applicable to an article of complicated form or large size, or to a ready-built structure. Studies have been undertaken to develop a method or a solder glass wherein the soldering can be performed at decreased temperatures. However, the studies have not resulted in satisfactory solder glasses. In addition, soldering metal articles with a solder glass to form an adhesive bond therebetween has not been attainable without decreasing the characteristic performance of the metals.
In short, despite repeated attempts in the prior art, there are no known heat-resistant inorganic compositions capable of providing a very firm and stable coating layer or adhesive layer within a temperature range from 100.degree. to 800.degree. C., which can be feasibly coated or bonded at room temperatures.
Japanese Patent Kokai No. 54-133526 proposes an inorganic adhesive agent composed of alumina and sodium silicate suitable for use in adhesive bonding of an aluminous ceramic ware and an iron plate. This adhesive has sufficient adhesive bonding strength, but the disadvantage of poor resistance against heat shock.
Further, Japanese Patent Kokai No. 55-29548 proposes that a composition comprising sodium silicate; an inorganic powder such as calcined clay, nitrides, carbides and metal powders; and an organic surface active agent is useful as an adhesive for bonding a metal and a ceramic by virtue of its capacacity to absorb the strain caused by the differences in the thermal expansions. This adhesive composition is defective because its use is limited due to the fact that the substrate to be bonded therewith cannot be heated at a temperature higher than the decomposition temperature of the surface active agent contained in the composition.
Japanese Patent Publication No. 56-391 proposes a protective coating composition for ferrous materials which contact molten metals. The composition disclosed therein is composed of an aqueous mixture of colloidal silica, sodium oxide, lithium oxide, glycerin, an anionic surface active agent and water, with an admixture of a ceramic powder as the aggregate. This composition is disadvantageous due to its extremely low stability when heated at high temperatures or subjected to rapid temperature changes.
Thus, a satisfactory heat-resistant inorganic composition suitable for coating or adhesive use has not been known hitherto.